1. Technical Field
The present invention relates to the field of desalination, and more particularly, to recovery of osmotic power.
2. Discussion of Related Art
In reverse osmosis (RO) desalination, water is extracted through a membrane from a pressurized feed, generating pressurized brine. The brine is both gauge pressurized (mechanical pressure) and osmotic pressurized (having a high salt concentration. Efforts have been to conducted to use the pressure of the brine to pressurize the feed, thereby recovering some of the high energy costs involved in pressurizing the feed.
Work or pressure exchangers (e.g. DWEER, ERI) are basically reciprocal pumping devices that are filled up by low pressure seawater in the first part of the cycle and are pressurized by valves exposed to high brine pressure in the second part of cycle. Work exchangers transform with high efficiency the gauge pressure of the brine to gauge pressure of the feed, as illustrated in FIGS. 1A and 1B.
FIGS. 1A and 1B illustrate a desalination system and a waste water treatment system according to prior art. The desalination system comprises a reverse osmosis (RO) unit (e.g. sea water reverse osmosis—SWRO) 80 receiving seawater through an input 81 and producing product water supplied to consumers 60 (via output 89). Seawater is pumped by main pump 82 to input 81, and the pressure of the generated brine (in output 88) is recovered by a work exchanger 90 that receives seawater and uses the brine gauge pressure (received at input 119) to pump the water through output 99 and an auxiliary pump 98 to input 81. Waste water from consumers 60 is collected (71) and is treated, e.g. by a membrane bioreactor 70 and further by sewage treatment facilities 61.
For example, Mediterranean seawater has 4% salinity and an osmotic pressure of approximately 30 bar. Seawater is pressurized to a gauge pressure of approximately 62 bar by high pressure pump 82 and passes along several RO membranes located in pressure vessels 80 from the feed entrance 81 to the brine outlet 88. As the seawater moves along the feed side of the membranes, about 50% of the seawater penetrates the RO membranes and becomes desalinated product (permeate) and the residual 50% exits the pressure vessels and accumulates in salt concentration (8% salinity), twice the salinity of the feed seawater (4% salinity), and an osmotic pressure of approximately 60 bar (at brine outlet 88). As the feed flows through pressure vessels 80, the gauge pressure drops slightly by 1.5% and remains at approximately 60 bar. Work exchanger 90 receives the gauge pressurized brine (at 119) and transmit the gauge pressure to the feed (at 99) with some loss of pressure. A low pressure pump 98 is used to compensate for losses in pipelines and pressure vessels, which is how the gauge pressure of the brine is recovered and transferred to pressurize the feed seawater (at 81), sparing much of the work of high pressure pump 82. This process has an extremely high efficiency of 96% for power transportation from the brine to the seawater feed stream.
While recovering the gauge pressure of the brine is carried out with high efficiency as explained above, recovering the osmotic pressure of the brine is currently in very initial stages of development.
For example, Statkraft, a leading player in Europe in renewable energy based out of Norway, implemented an Osmotic Power Generation system based on penetration of river water to ocean water through semi-permeable membranes by forward osmosis (FO, see below).
River water has a low osmotic pressure (POr) of 0.3 bars and a low gauge pressure (PGr) of 5 bars). Ocean water has a higher gauge pressure (PGo) of 30 bars and a natural osmotic pressure (POo) of 30 bars. The high osmotic pressure of the ocean water allows river water to penetrate the RO membranes, producing energy that can be recovered by a hydraulic turbine and converted to electricity by an electrical generator. However, this method is characterized by the following disadvantages: (1) Water treatment—Both the river and ocean water have to be intensively cleaned to remove all suspended solids. (2) Pumping Energy—Ocean water has to be pumped to a pressure of about 30 bars and all flow restrictions including piping, pipe fittings and equipment losses associated with this transfer must be considered. (3) Energy Transfer Efficiency—There are large energy losses associated with the recovery of energy via turbine and electrical generator, for example, combining 1 m3/s of ocean water with 1 m3/r river water through semi-permeable membranes produces approximately 410 KWh (calculation basis: the above mentioned pressures, 85% pumping efficiency of sea water and river water, 97% motor efficiency result in pre-pressure power consumptions of 3639 KWh for ocean water and 606 KWh for river water, which are reduced from an energy gain of 4665 KWh from the process, assuming 88% turbine efficiency and 96% generator efficiency).
Forward osmosis (FO) is a concentration-driven membrane process, which utilizes the osmotic pressure difference across a selectively permeable membrane as the driving force for the transport of water through the membrane, from a low osmotic pressure “feed” solution into a concentrated “draw” solution having high osmotic pressure. Different applications and implementations of the process were suggested over the years. The process that counteracts Forward Osmotic Process is concentration polarization, as illustrated in FIGS. 1C-1E. Concentration polarization (CP) is an important issue in forward osmosis processes and it is coupled effect of internal dilutive CP (IDCP) and external concentrative CP (ECCP), or Dilutive External CP (DECP) and Internal Concentrative CP ICCP (FIG. 1E). The different type of concentration polarization comes from different scheme of draw solution movement near the support layer of near salt rejection skin. Those types of CP limit FO flux and efficiency of the process to such extend that making it not cost effective.
Compared with pressure-driven RO membrane processes, the FO process exhibits unparalleled advantages of nearly no hydraulic pressure operation, nearly complete rejection of many contaminants, and potentially low membrane fouling tendency. As a result, FO has received intensive studies recently for a range of potential applications, which include wastewater treatment, water purification, seawater desalination, as well as power generation. However, there exist a number of technical barriers that impede FOs industrial applications, a main technical barrier is Concentration Polarization which is explained below.
RO membranes have an asymmetric structure, typically consisting of a thin selective active layer supported by thick layers of porous polymer and fabric termed “Support Layer”. The membranes used by the FO process are, in principle, similar to those used in RO, in that the properties of the rejecting layer of the membranes may be identical. FO membranes, however, differ in the thickness of the support layers, which must be much thinner to diminish concentration polarization barrier effect.
FIGS. 1C-1E illustrate the buildup of concentration polarization across a FO membrane, according to the prior art. FIG. 1C presents experimental results illustrating the reduction in the efficiency of the FO process over time, FIG. 1D illustrates schematically an overall product gain 50 in the FO process under action of CP, and FIG. 1E schematically illustrates the buildup of CP across the membrane.
FIG. 1C illustrates two experiments in which waste water had 0.1% salinity, brine 7.7% salinity, temperature ranged 30-34° at two flow rates: (1) 160-170 liter/hour at 5.1-8 bar, (2) 74-78 liter/hour at 2.7-4.8 bar, and an average pressure. The flux through the membrane decreased from 29 Liters per square Meter per Hour (LMH) at the commencement of operation to ca. 2.5 LMH at steady state. FIG. 1D is a schematic representation of the first 15 minutes of the graph of FIG. 1C.
Feed flow moving by osmotic forces in to draw solution via support 116 and membrane rejection layer 115 leaves salt in support layer 116. This feed water movement provides two effects: The First effect is salt concentration in side support layer ICCP. The Second effect is EDCP. From beginning of FO process the flux is quit high 30 LMH and it takes about 6-9 minute to develop full ICCP. When the ICCP process is fully developed flux is diminished to about 2.5 LMH.
Internal and external concentration polarization (ICP and ECP, FIG. 1E) occur during the mass transport process, and significantly reduce the available osmotic driving force. The concentration polarization phenomena are associated with solute physical properties, fluid dynamics and, most significantly, the membrane structure. The ECP can be controlled hydro dynamically but the ICP occurs in the porous support layer of the membrane, making it difficult to handle. A main effort in the FO industry is to produce a membrane that minimize the ICP.
FIG. 1E illustrates the buildup of concentration polarization in the prior art. Stages (a)-(d) illustrate the concentration gradient across membrane 115 and support layers 116A, 116B at sequential time points along the first ca. 10 minutes of the graphs presented in FIGS. 1C and 1D. (a) illustrates the initial operation of the FO process, as the full baseline NDF drives water extraction. (b) and (c) illustrate consequent intermediate stages in the build up of CP, namely the accumulation of solutes at the feed side 118 of membrane 115 and the dilution of the draw solution at the draw side 117 of membrane 115. The actual NDF arises only from the difference between the reduced concentration gradient in support layers 116A, 116B across membrane 115. (d) illustrates the final CP across membrane 115, that actually dominates prior art FO processes as it occurs from ca. 10 minutes after activation of the FO process, in which the actual NDF is minimal.